Finely divided particles of silver are widely used in the electronics industry for the manufacture of thick film pastes which can be used to form conductive circuit patterns in a wide variety of electronic components. In such applications, the thick film pastes are applied by such methods as screen printing and are either dried to remove solvent or dried and fired to effect volatilization of the liquid organic medium from the paste and to effect sintering of the silver particles to form an integral circuit pattern.
Experience in the use of such thick film pastes has taught that conductivity of the final dried and/or fired pastes and thus the specific conductivity of the circuits prepared therefrom can be substantially improved by the use of flake silver particles. For this reason, most of the thick film pastes used throughout the electronics industry employ flakeshaped particles of silver rather than spherical or semi-spherical particles.
Flake silver particles are mainly formed by ball-milling more spherical particles. However, in the ball-milling operations it is necessary in most instances to add to the mass to be milled an amount of surfactant sufficient to prevent the quite maleable silver particles from undergoing cold welding (fusion) as their configuration is changed to flake morphology. The surfactants most used for this purpose are ionic surfactants and particularly anionic surfactants such as the sodium salts of various longchain fatty acids. In particular, the sodium salts of lauryl, palmitic and stearic acids are frequently used.
When ionic surfactants such as those described above are used in the preparation of flake silver particles, the resultant flakes usually contain substantial amounts of sodium ions, which are harmful impurities in many electronic systems prepared from thick film pastes. Some of the ionic impurities are chemisorbed on the flake particles. However, most of the ionic impurities associated with the silver flake particles are in the form of a micelle surrounding a core of silver metal. This micelle of excess surfactant is firmly attached to the silver and acts as a trap which inhibits the removal of the sodium ions from the powder by washing with water. As an illustration of this phenomenon, a typical commercial flake silver product has been found to contain 160-200 ppm residual extractable sodium. Even after repeated washings with de-ionized water, it is reduced to only 50-70 ppm sodium.
Heretofore, it has not been possible substantially to lower the ion content of flake silver without altering the configuration of the particles. Therefore, in view of the increasing need for conductive materials having very low contaminant levels, there is a very real need for flake silver particles having very low ionic contamination. To accomplish this, a process is needed by which surfactant-treated flake silver particles can be de-ionized without adversely affecting the physical characterisitics of the flakes.